The present invention relates generally to magnetic resonance (MR) imaging systems. More particularly, the present invention relates to an MR imaging system equipped for real-time imaging and which allows interactive modification of the image contrast of MR images produced therein.
When an object of interest, such as human tissue, is subjected to an uniform magnetic field (polarizing field Bo along the z direction in a Cartesian coordinate system denoted as x, y, and z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the object, or tissue, is subjected to a magnetic field (excitation field B1) which is the x-y plane and which is near the Larmor frequency, the net aligned moment, M2, may be rotated, or xe2x80x9ctippedxe2x80x9d at a certain tipping angle, into the x-y plane to produce a net transverse magnetic moment M1. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an MR image.
When utilizing these signals to produce MR images, magnetic field gradients (Gx, Gy and Gz) are also employed. Typically, the object to be imaged is scanned by a sequence of measurement cycles in which these gradient waveforms vary according to the particular localization method being used. The resulting set of received NMR signals (also referred to as MR signals) are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
When viewing an MR image of a structure of interest, such as an anatomical section, the MR imaging system operator may desire to view an MR image in which one or more types of tissue comprising the anatomical section is contrasted with respect to the remaining types of tissue comprising that anatomical section. Moreover, the operator may desire to modify the image contrast of an MR image acquisition in progress or to prescribe the image contrast prior to an MR image acquisition.
Each MR pulse sequence responsible for an MR image is comprised of at least one set of (regular) waveform segmentsxe2x80x94the imaging waveform segments. In addition, the MR pulse sequence includes certain features or architecture to provide image contrast in the MR image: (1) image contrast mechanisms can be inherent in the imaging waveform segments; (2) one or more parameters associated with the MR pulse sequence can be modified and/or specified by the operator, thereby affecting image contrast; or (3) one or more sets of image contrast waveform segments can be included along with the imaging waveform segments to comprise the MR pulse sequence. In this last case where image contrast waveform segments are utilized, such image contrast mechanisms are made possible by a corresponding magnetization preparation applied to the anatomical section prior to the application of the imaging waveform segments. Briefly, magnetization preparation involves preparing the spin state in the bore such that the anatomical section to be imaged is in a certain magnetized state immediately before the regular image scanning commences.
In conventional MR imaging systems, every MR pulse sequence responsible for a specific image contrast is typically constructed and stored in the MR imaging system prior to scanning. For example, an MR pulse sequence may comprise a specific image contrast waveform segment permanently linked to an imaging waveform segment. Then when the operator desires this specific image contrast, this all-inclusive pulse sequence is evoked and executed in its entirety. In another example, the MR pulse sequence may be constructed prior to scanning from a specific selection of short (or more basic components comprising the) waveform segments.
The drawback to these types of pulse sequence architectures is that the operator must wait until the image acquisition in progress is completed before newly desired image contrast mechanism(s) can be evoked. Moreover, even if the amplitudes, periods, or other parameters relating to a portion of die MR pulse sequence (e.g., the image contrast waveform segment) can be modified while the image acquisition is in progress (e.g., amplitude is set to zero), there is only negligible reduction in acquisition time because the modified portion of the pulse sequence must still be executed along with the rest of the pulse sequence.
Thus, there is a need for an MR imaging system capable of providing interactively prescribable image contrast mechanisms in real-time. There is a further need for an MR imaging system capable of modifying image contrast mechanisms in MR pulse sequences, as desired, through dynamic construction of MR pulse sequences of run-time.
One embodiment of the invention relates to a method for interactively prescribing an image contrast of a magnetic resonance (MR) image produced in a magnetic resonance (MR) imaging system. The method includes storing a plurality of waveform segments in a waveform memory. Each of the waveform segments is associated with a distinct memory address and at least one of the waveform segment includes an image contrast mechanism. The method further includes selecting from the plurality of waveform segments stored in the waveform memory, and constructing an MR pulse sequence in real-time by dynamically connecting selected waveform segments at run-time. The method further includes acquiring MR data in real-time by execution of the MR pulse sequence to generate a current MR image that includes a desired image contrast.
Another embodiment of die invention relates to an interactive magnetic resonance (MR) imaging system. The system includes means for storing a plurality of waveform segments, and means for selecting from the plurality of waveform segments. Each of the waveform segments is associated with a distinct memory address and at least one of the waveform segment includes an image contrast mechanism. The system further includes means for constructing an MR pulse sequence in real-time by dynamically connecting selected waveform segments at run-time. The system further includes means for acquiring MR data in real-time by execution of the MR pulse sequence to generate a current MR image that includes a desired image contrast.
Another embodiment of the invention relates to an interactive magnetic resonance (MR) imaging system. The system includes a memory configured to store a plurality of waveform segments. Each of the waveform segments is associated with a distinct memory address and at least one of the waveform segment includes an image contrast mechanism. The system further includes an operator interface configured to permit an operator to select from the plurality of waveform segments, and a sequence controller coupled to the memory and configured to dynamically connect the selected waveform segments at run-time to construct a MR pulse sequence. The system further includes a MR imaging device coupled to the operator interface, memory, and sequence controller and configured to acquire MR data in real-time and generate a current MR image that includes a desired image contrast.